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Cell A: 2 

Cell B: 4

Certain chemical components act as inhibitors of the mitotic cell division and they are called mitotic poisons.

Meiotic division leads to genetic variability.

(d) Colchicine

(d) Kinetochore

(c) Pachytene

(b) Pachytene

(d) Movement of chromosome towards pole

(c) Coloured bodies

(a) Metaphase. 

(b) 4 

(c) A – Animal 

B – Animal 

C – Plant 

(iv)All the three

The exchange of chromatids between homologous chromosomes is called crossing-over. This is the process by which the two chromosomes of a homologous pair exchange equal segments with each other.

Crossing over occurs in the first division of meiosis. At that stage each chromosome has replicated into two strands called sister chromatids. The two homologous chromosomes of a pair synapse, or come together. While the chromosomes are synapsed, breaks occur at corresponding points in two of the non-sister chromatids, i.e., in one chromatid of each chromosome.

Since the chromosomes are homologous, breaks at corresponding points mean that the segments that are broken off contain corresponding genes, i.e., alleles. The broken sections are then exchanged between the chromosomes to form complete new units, and each new recombined chromosome of the pair can go to a different daughter sex cell. It results in recombination of genes found on the same chromosome, called linked genes that would otherwise always be transmitted together.

(c) Anaphase, telophase

(a) Somatic (body) 

(b) Four 

(c) Reproductive 

(d) 23 and 23 

(e) Homologous 

(f) centriole

Mitosis is a process of cell division, in which chromosomes are equally distributed into two daughter cells so it is equational division. 

The chromosome number in daughter cells is equal to that of the parent cell.

Meiosis is called as reductional division because the chromosome number gets reduced to its half whereas mitosis is equational division because the chromosome number remains the same after division.

(a) – (iii)

(b) – (v)

(c) – (i)

(d) – (ii)

(e) – (iv)

The number of chromosomes in the parent and daughter cells is the same in mitosis. Hence it is called equational division. 

1. Metaphase

2. Anaphase

3. Zygotene

4. Pachytene

(a) Metaphase 

(b) Anaphase 

(c)  Zygotene 

(d) Pachytene

G₁ stage, ‘S’ stage and G₂ stage.

In animal cell, cytokinesis is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.

Due to the presence of cell wall, cytokinesis is different in plants. In plants, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell plate that represents the middle lamella between the walls of two adjacent cells. Organelles like mitochondria and plastids get distributed between the two daughter cells during this process.

Animal cytokinesis : Appearance of furrow in plasma membrane which deepens and joins in the centre dividing cell cytoplasm into two. 

Plant cytokinesis : Formation of new cell wall begins with the formation of a simple precursor − cell plate which represents the middle lamella between the walls of two adjacent cells.

In animal cell, a furrow appears in the plasma membrane. The furrow gradually deepens and finally joins in the centre. Thus, the cytoplasm is divided into two parts. In plant cells, cell wall formation begins in the centre. This grows outwards to meet the existing lateral walls and thus, the cytoplasm is divided into two parts.

The four daughter cells formed after microsporogenesis in flowering plants are equal in size. The four daughter cells formed after megasporogenesis in flowering plants are unequal in size. 

The centromere splits during anphase of mitosis, while it does not split during anaphase I of meiosis.

Depending upon the stage, when meiosis occurs, the meiosis is of three types : Gametic, zygotic and sporic. 

(i) Gametic or terminal meiosis : In most animals and some lower plants, meiosis takes place during the formation of gametes (gametogenesis). Such a meiosis is described as gametic or terminal meiosis. When two gametes fuse in fertilization, the diploid zygote is formed. Gametic meiosis results in diplontic life cycle. 

(ii) Zygotic or initial meiosis: In some lower plants meiosis takes place in the zygote and the resulting organisms are haploid. It is called zygotic or initial meiosis. Organisms having zygotic meiosis have haplontic life cycle. 

(iii) Sporic or intermediate meiosis: In most of the plants, meiosis occurs at the time of sporogenesis. It is called sporic or intermediate meiosis. Spore gives rise to a new gametophytic phase in the life cycle. The gametophyte produces gametes. The life of plant is said to be diplo-haplontic because of the presence of diploid and haploid multicellular phases.

(i) Formation of gametes, Genetic variability Maintenance of chromosomal number 

(ii) Anaphase of mitosis Centromeres and chromatids separate; chromatids moves to opposite poles due to shortening of spindle fibres. 

Anaphase I of meiosis: Homologous chromosomes separate while chromatids remain associated at their centromeres. 

(iii) Pairing of homologous chomosomes is synapsis. It is observed during zygotene stage. 

• Conservation of specific chromosome number of each species across generations is achieved by meiosis.
• Due to crossing over, the hereditary characters from male and female parents get mixed. So genetic variability in the population of organisms from one generation to the next occurs which is most important for the process of evolution. 
• Gametes are produced by Meiosis.
• Meiosis results in reproduction and is similar among different organisms.

Meiosis is called as reduction division because the chromosome number gets reduced to its half. It helps in producing haploid gametes in sexually reproducing organisms.

Anaphase   I

During anaphase of mitosis. 

Mitosis, because it keeps all the somatic cells of an organism genetically similar, so that they are able to regenerate a part or whole of the organism.

Dissolution of synaptonemal complex occurs and the recombined chromosomes separate from each other except at the sites of crossing over. These X-shaped structures are called chiasmata.

Significance of Chiasmata : The genetic constitution of the daughter cells differs from the parent cells due to crossing over. There is a mixture of paternal and maternal gene in each chromosome of daughter cells mostly.

• In-plant cell, Asters are not formed. In animal cell Asters are formed.
• In-plant cell, cytokinesis occurs due to the formation of a cell plate. In an animal cell, cytokinesis occurs due to the formation of a cleavage furrow.

Enzyme that help in breakage is endonuclease and enzyme that help in reunion is R-protein.

Kinetochores are small disc-shaped structures at the surface of centromeres. They serve as the binding site for the spindle fibres to the centromeres of chromosomes.

When nuclear division takes place without cytoplasmic division it results in the formation of large number of nuclei in a cell. This condition is called syncytium.

Small disc-shaped structures at the surface of the centromeres which serve as the sites of attachment of spindle fibres are called as kinetochores.

Contraction of chromosome fibres, formation and expansion of interzonal fibres and poleward dissolution of microtubules of chromosome fibres.

When cell enters in G₀ cycle.

Kinetochore is a part of a chromosome for the attachment of chromosomal fibres.

The time interval between two cell cycles is called generation time.

It is made up of two lateral arms and a medium element, which forms pairing of homologous chromosomes.

It is made up of two lateral arms and a medium element, which forms pairing of homologous chromosomes.

Rungs of DNA ladder is made of nitrogenous bases which includes Adenine (A), Guanine (G), Cytosine (C) and Thymine (T).

(a) 2 

(b) 2 on each strand 

(c) 1- Phosphate, 

2- Sugar, 

3- Nitrogen Base, 

4- Hydrogen Bond, 

5 – Base 

(d) Nucleotide